This invention relates to a magnetic recording medium for recording information and, in particular, to a magnetic disk to be mounted to a HDD (Hard Disk Drive) and the like.
In a conventional magnetic disk drive, use has been made of a CSS (Contact Start and Stop) system. In the CSS system, a magnetic recording head rests on a surface of a magnetic recording medium in contact with its inner-diameter area as a contact sliding zone when the rotation of the magnetic recording medium is stopped. During start-up, the magnetic recording head slides over the contact sliding zone in contact therewith to be slightly lifted up. When the magnetic recording head reaches a read/write zone located outside the contact sliding zone, a read or a write operation is started. In the above-mentioned CSS system, the contact sliding zone is required in addition to the read/write zone.
In the CSS system, the magnetic recording medium is provided with a roughened surface, called a textured surface, having a predetermined surface roughness in order to avoid adhesion between the magnetic recording medium and the magnetic recording head when the rotation of the magnetic recording medium is stopped.
In the CSS system, the magnetic recording medium must be protected from a damage resulting from contact sliding (or frictional sliding) of the magnetic recording head. To this end, the surface of the magnetic recording medium is coated with a carbon-based protection layer and a lubrication layer is applied onto the carbon-based protection layer as disclosed in Japanese patent No. 3058066.
On the other hand, use is recently made of a LUL (Load Unload) system allowing a remarkable increase in storage capacity. In the LUL system, the magnetic recording head rests on an inclined support, called a ramp, located outside the magnetic recording medium when the rotation of the magnetic recording medium is stopped. During start-up, the magnetic recording head slides from the ramp to an area above the surface of the magnetic recording medium after the rotation of the magnetic recording medium is started. Thus, in the LUL system, the magnetic recording head does not slide over the magnetic recording medium in contact therewith.
In the LUL system, it is unnecessary to provide the magnetic recording medium with the above-mentioned contact sliding zone required in the CSS system for the magnetic recording head to slide thereon. Therefore, the LUL system is advantageous in that the read/write zone of the magnetic recording medium is widened and the storage capacity of the magnetic recording medium is increased as compared with the CSS system.
In the LUL system, the magnetic recording medium and the magnetic recording head do not contact with each other. Therefore, the magnetic recording medium need not have the textured surface required in the CSS system and is improved in smoothness. As a consequence, it is possible to achieve a low flying height (15 nm or less) of the magnetic recording head as compared with the CSS system and to thereby improve the recording density of the magnetic recording medium.
With the transition from the CSS system to the LUL system, however, the magnetic recording head suffers corrosion of a read element portion thereof and frequent occurrence of fly stiction during operation at a flying position.
Upon occurrence of corrosion in the read element portion of the magnetic recording head, an output level of a readout signal is decreased. In this event, reading errors frequently occur and, sometimes, a reading operation is completely impossible. A corroded part may be enlarged to damage the magnetic recording medium while the magnetic recording head is operated at a flying position.
The fly stiction is a phenomenon that, while the magnetic recording head is operated at a flying position, its flying posture and/or its flying height is varied. This results in frequent occurrence of irregular variation in output level of the readout signal. Sometimes, the magnetic recording head at a flying position is brought into contact with the magnetic recording medium to cause crash so that the magnetic recording medium is broken. The fly stiction often occurs suddenly without any sign or precursory condition and is one of defects which are difficult to control.
As a result of research, the present inventors have clarified the factors causing frequent occurrence of the above-mentioned defects in the LUL system.
In recent years, the magnetic recording head uses a NPAB slider (Negative Pressure Air Bearing slider) easily controlled in flying height. In the magnetic recording head of the type, a negative pressure is produced on a slider surface during operation at a flying position. On the other hand, a very small amount of organic and inorganic substances are deposited in the read/write zone on the surface of the magnetic recording medium as deposited substances. Under the above-mentioned negative pressure, the magnetic recording head gradually collects, like a cleaner, the deposited substances and the lubrication layer comprising a highly flowable lubricant. The deposited substances thus collected are condensed and deposited on the slider surface as contaminants.
As a result of research by the present inventors, it has been found out that the CSS system has a cleaning function of cleaning these contaminants migrating from the magnetic recording medium to the magnetic recording head. Specifically, when the magnetic recording head slides over the contact sliding zone on the surface of the magnetic recording medium in contact therewith, the contaminants are removed and the magnetic recording head is cleaned via the contact with the surface of the magnetic recording medium. On the other hand, the above-mentioned cleaning function is not achieved in the LUL system because the magnetic recording head does not slide over the magnetic recording medium in contact therewith.
The inventors have found out that, in the LUL system without the cleaning function, the contaminants migrating to and condensed in the magnetic recording head, particularly, acidic contaminants such as sulfate contaminants, chloride contaminants, and nitride contaminants cause corrosion of the read element portion. In particular, a magnetoresistive read element (for example, MR, GMR, TMR elements) capable of obtaining a high output level is susceptible to corrosion.
Unlike a thin-film head which has traditionally been used, the magnetoresistive head has a read/write (R/W) separate structure in which a write element and a read element are separated. In case of the read/write separate structure, it is necessary to form a wide shield of, for example, Permalloy such as Fe—Ni, between the read and the read elements. Since Permalloy is an alloy susceptible to corrosion, the magnetoresistive head must strictly be protected from corrosion, unlike the thin-film head.
Since the above-mentioned cleaning function is not achieved in the LUL system, the lubricant forming the lubrication layer of the magnetic recording medium tends to migrate and be deposited on the magnetic recording head. The lubricant of the magnetic recording medium, thus migrating and deposited onto the magnetic recording head, tends to seriously disturb the flying posture of the magnetic recording head, resulting in occurrence of the fly stiction.
If the amount of those deposited substances or contaminants on the magnetic recording head reaches a predetermined level, the deposited substances will fall down onto the magnetic recording medium to cause crash while the magnetic recording head is operated at a flying position.
Furthermore, the improvement in smoothness of the surface of the magnetic recording medium following the transition into the LUL system further increases the mobility of a small amount of the organic and the inorganic substances deposited on the surface of the magnetic recording medium as well as the mobility of the highly flowable lubricant so that the above-mentioned migration is promoted. It has also been found out that the decrease in flying height of the magnetic recording head also promotes the above-mentioned migration.
From the above-mentioned reasons, a low flying height not greater than 15 nm can not stably be achieved in the LUL system unless the above-mentioned migration is suppressed.